Offshore drilling operations have been undertaken for many years. Traditionally, pressure within a drill string and riser pipe have been governed by the density of drilling mud alone. More recently, attempts have been made to control the pressure within a drill string and riser pipe using methods and characteristics in addition to the density of drilling mud. Such attempts may be referred to in the art as managed pressure drilling (MPD). See, e.g., Frink, Managed pressure drilling-what's in a name?, Drilling Contractor, March/April 2006, pp. 36-39.